The dynamic role of Earth's continental mantle in ‘deep time’ volatile cycles

Continental mantle represents one of Earth’s most ancient and long-lived chemical reservoirs. It plays a crucial role in the global cycling of volatile elements—such as C, H, S, F  and Cl —because of its unique ability to both sequester volatiles via metasomatism and release them to the atmosphere during volcanism (Gibson and McKenzie, 2023).

The widespread generation of deep-sourced, volatile-rich melts is borne out by global maps of magmas rich in CO₂, H₂O, S and F (e.g. kimberlites, lamproites and carbonatites). Moreover, mantle xenoliths preserve evidence of repeated episodes of pervasive, reactive percolation and stalling of these volatile-rich melts. High-precision analyses of volatile elements in the abundant nominally-volatile-free mantle minerals and accessory phases, together with analyses of volatiles of intraplate magmas, allow quantification of the storage of volatile elements in the lithospheric mantle.

Recent advances in global tomography, particularly multi-mode surface wave analysis, have significantly refined estimates of lithospheric thickness. These improvements enable more reliable calculations of lithospheric mantle volume across different geodynamic environments, including cratonic regions, continental off-craton areas and oceanic domains. The results indicate that the most significant global volatile reservoir resides within the mantle beneath ancient cratons. This is primarily due to their large volume and the elevated volatile concentrations preserved within their stable ‘roots’. Our new thermal models show that the outer ~ 50 km of craton margins is especially susceptible to devolatilisation during rifting and heating events (Gibson et al., 2024b). The thermal stability of craton interiors, however, ensures these regions have acted as long-term volatile sinks for at least the past 2.5 billion years. The volatile budget of off-craton lithospheric mantle is more dynamic. Volatiles stored in these regions may have significantly shorter residence times and can be rapidly remobilized through rifting and heating events. As a result, off-craton lithospheric mantle can transition from a volatile ‘sink’ to a ‘source’ over relatively short geological timescales, potentially within a few million years.

The ultimate source of volatiles stored in the continental mantle is challenging to decipher but ³He/⁴He exhibits a systematic behaviour with melt depletion in mantle peridotites and deviations from this global trend may be correlated with subduction events (Gibson et al., 2024a). The dynamic nature of volatile storage and release within Earth's lower lithospheric ‘lid’ underscores the need for continued refinement of mantle volatile estimates to improve our understanding of deep volatile cycling.

 

Gibson, S. A., Crosby, J. C., Day, J. A. F., Stuart, F. M., DiNicola, L. & Riley, T. R. (2024a). Systematic behaviour of ³He/⁴He in Earth’s continental mantle. Geochimica et Cosmochimica Acta 384, 44–64.

Gibson, S. A. & McKenzie, D. (2023). On the role of the lithospheric mantle in global volatile cycles. Earth and Planetary Science Letters 602, 117946.

Gibson, S., McKenzie, D. & Lebedev, S. (2024b). The distribution and generation of carbonatites. Geology 52, 667–671.